A subtle move to gain access to the e4-square.
4... c7c6 Forever preventing Nb5, but the position of the knight on b6 is weakened. 5. c1e3 Immediately exploiting the problems with Black's last move. 5... g7g5 The best chance to create counterplay. Caruana undoubtedly had prepared this move in advance. 6. f4g5 Carlsen accepts the pawn sac. 6... e7e5 The idea with the sac is to control the center with the pawns. Next is d7-d5.